An understanding of the biology of human stem cells is of primary importance in attempting to address diseases of hemopoietic tissues. In order to study the properties, regulation, and differentiation decisions of human stem cells, we propose a multiparameter approach using both in vitro and in vivo models. The first part of the application, (SA#l) is aiming at the characterization of the functional features of integrin expressing subsets of human fetal stem cells that we have recently isolated (including the hierarchical order in the expression of integrins in relation to stem cell CD34 antigen expression, and their cytoadhesive interactions with stromal cells). In the second part of the application (SA#2), we will use a newly developed immunodeficient mouse model (bg/SCID) and several experimental innovations (implantation of human stroma, supply of unique cytokines) for studying the in vivo behavior of human stem cells. Parameters necessary for multilineage reconstitution by donor human cells, their "homing' ability, or the consequences of abrogation of their adhesive properties will be explored. The third and fourth parts of the application (SA#3 and SA#4) concern fundamental questions on the molecular control of early stages of hemopoiesis. Unique cell lines which differentiate in response to physiologic growth factors, as well as isolated populations of normal progenitors, will be employed. We will test whether multipotent single progenitors express a combination of lineage specific RNAs or whether the two daughter cells are phenotypically identical; in regards to erythroid cell differentiation we will examine whether late lineage specific RNA (globin mRNA) is present onprimitive BFUe and, if not, whether erythropoietin, a lineage specific regulator can induce directly the expression of this RNA (globin mRNA) and whether cell division is required for this purpose.